Friction lining material comprising aramid fibers with low degree of fibrillation and synthetic graphite

Abstract: The present invention relates to a fibrous base material comprising fibrillated aramid fibers with low degree of fibrillation, synthetic graphite and at least one filler material, and a non-asbestos friction material produced therefrom. In certain embodiments, the fibrous base material is impregnated with a phenolic or phenolic-base resin material, including, for example, a mixture of a phenolic resin and a silicone resin.

Description: The present invention relates to a fibrous base material comprising less fibrillated aramid fibers, manufactured or synthetic graphite and a filler material, such as diatomaceous earth. The invention further relates to a composite friction material comprising the above described fibrous base material impregnated with a phenolic resin or a modified phenolic resin blend. In certain embodiments, at least one silicone resin is blended with at least one phenolic resin for use in impregnating a fibrous base material. The friction material of the present invention has higher friction stability and thermal capability than conventional friction materials. Also, the resulting friction material is especially useful in high energy applications. New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems. In particular, a new high energy type friction material is needed. The new high energy friction material must be able to withstand high speeds wherein surface speeds are up to about 65m/seconds. Also, the friction material must be able to withstand high facing lining pressures up to about 1500 psi. It is also important that the friction material be useful under limited lubrication conditions. The friction material must have high heat resistance in order to be useful in the advanced transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions. The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize "shuddering" of materials during braking or the transmission system during power shift from one gear to another. Previously, asbestos fibers were included in the friction material for temperature stability. For example, the Arledter et al. U.S. Patent No. 3,270,846 patent describes phenolic and phenolic-modified resins used with asbestos. Now, however, due to health and environmental problems, asbestos is no longer being used. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by modifying impregnating paper or fiber materials with phenolic or phenolic-modified resins. These friction materials, however, do not rapidly dissipate the high heat generated, and do not have the necessary heat resistance and satisfactory high coefficient of friction performance now needed for use in the high speed systems currently being developed. While phenolic resins have found use in friction materials for wet clutch applications, the phenolic resins have various limitations. The phenolic resin friction materials do not have the high heat resistance necessary for use with the new high energy transmission systems. In particular, the phenolic resins carbonize at a temperature of about 450° to 500°C which is too low to be useful in high energy applications. In addition, phenolic resins are rigid materials and when the phenolic resins are used in a friction material, uneven lining wear and separator plate "hot spots" result. Attempts to overcome the limitations and drawbacks of phenolic resin friction materials include the replacement of phenolic resins with other thermosetting resins. One attempt to produce friction materials involves the modification of a phenolic resin with various synthetic resins. One example, described in Takarada et al. U.S. Patent No. 4,657,951, is a phenolic resin modified with an organopolysiloxane which is compression molded to form a friction material. The phenolic resin and organopolysiloxane are reacted together to effect a condensation reaction which is then distilled, solidified by cooling, and pulverized to obtain a powdered phenolic-modified resin. The powdered phenolic-modified resin was used in forming a compression molded friction material. As far as is known, there is no disclosure of a friction material for use in transmission systems which includes a silicone material blended with a phenolic material and used to impregnate a friction paper. While the Hartmann et al. U.S. Patent No. 3,911,045 reference discusses a silicone material blended with phenolic resins for use as a compression molding composition, there is no disclosure or suggestion that a silicone material could successfully be blended with a resin material and used to impregnate a friction lining material. On the contrary, previous attempts to use silicone resins in friction materials have been unacceptable. A friction lining that is impregnated or saturated with a silicone resin has, in the past, demonstrated poor shear strength and delamination resistance. Further, friction materials saturated with a silicone resin are usually too elastic and therefore tests with undesirable friction and wear characteristics resulting. It is not surprising that molded friction lining compositions formed entirely of a phenolformaldehyde resin-polysiloxane resin have not been used even though they are known, since such molded compositions do not have the necessary constant coefficient of friction characteristics and such friction materials fail under high energy and high heat conditions. GB-A-2224285 discloses a friction material containing 0.85 to 30 wt% of carbon fibers and 2 to 20 wt% of aramid fibers. The aramid fibers preferably consist of para-aramid fibers and/or a combination of chopped aramid fibers and fibrillar aramid fibers. US-A-4743634 discloses a non-asbestos friction element comprising a fibrous material and a small amount of diatomite having a particle size including agglomerates such that at least 10 wt% thereof is maintained on a 325 mesh screen. In order for friction materials to be useful in "wet" applications, the friction material must have a wide variety of acceptable characteristics. The friction material must be resilient or elastic yet resistant to compression set, abrasion and stress; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not met, optimum performance of the friction material is not met. Thus, it is also important that the impregnating resin be used with a suitable friction lining or fibrous base material to form a high energy application friction material. The friction material must have good shear strength both when saturated with the wet resin during impregnation and when saturated with brake fluid or transmission oil during use. It is also important, under certain applications, that the friction material have high porosity such that there is a high fluid permeation capacity during use. Thus, it is important that the friction material not only be porous, it must also be compressible. The fluids permeated into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission, yet the lining material must not collapse. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission. Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art. A further object of this invention is to provide friction materials with high thermal conductivity, porosity and strength. As a result of extensive research in view of the need for a better friction material, a friction material with improved characteristics has been developed by the invention. The present wet friction material is useful in "wet" applications where the friction material is "wetted" or impregnated with a liquid such as brake fluid or automatic transmission fluid during use. During use of the "wet" friction material, the fluid is ultimately squeezed from or is impregnating the friction material. Wet friction materials differ greatly, both in their compositions and physical characteristics from "dry" friction materials. In order to achieve the requirements discussed above, many materials were evaluated for friction and heat resistant characteristics under conditions similar to those encountered during operation. Both commercially available brake linings and transmission materials were investigated and proved not to be suitable for use in high energy applications. The present invention is especially useful in brakes and in clutch applications. The present invention provides a fibrous base material for use in a non-asbestos friction material comprising a plurality of fibrillated aramid fibers and at least one filler material, characterised in that the material further comprises synthetic graphite, and in that the aramid fibers have a freeness of at least 450 on the Canadian Freeness Standard Index. Other ingredients may also be provided in the fibrous base material. The fibrous base material can be impregnated using different resin systems. In certain embodiments, it is useful to impregnate the fibrous based material with a phenolic resin or a modified phenolic-based resin. It has now been discovered that, in certain embodiments, when a silicone resin is blended or mixed with a phenolic resin in compatible solvents and that silicone-phenolic resin blend is used to impregnate a fibrous base material of the present invention, a high energy friction material is formed. Such high energy friction material has high friction stability and high heat resistance. The friction material of the present invention prevents uneven lining wear and therefore the formation of separator plate "hot spots" from developing during the useful life of the friction material. When there is little uneven wear on the friction material, there is more likelihood to maintain "steady state" of the clutch or brake components and therefore, more consistent performance of the clutch and brake. Further, the friction material of the present invention shows good shear strength such that the friction material resists delamination during use. Reference in now made to the accompanying drawings, in which : Fig. 1A is a scanning electron microphotograph of a fibrous base material impregnated with a silicone-phenolic blend (Example C). Fig. 1B is a scanning electron microphotograph of a conventional fibrous material impregnated with a phenolic resin (Conventional - 1). Fig. 2 is a thermal gravimetric analysis (TGA) graph showing the relationship between the percent of weight change and increases temperatures for a fibrous base material impregnated with a phenolic resin (Example A) or a fibrous base material impregnated with a silicone-phenolic resin blend (Example B). Fig. 3 is a TGA graph showing the percent of weight loss as temperatures increase, the change in the derivative weight (%/°C), and the amount and percent of residue for Example A shown in Fig. 2. Fig. 4 is a TGA graph showing the percent of weight loss as temperatures increase, the change in the derivative weight, and the amount and percent of for Example B shown in Fig. 2. Fig. 5 is a graph showing the stop time in seconds as the number of cycles increases for a conventional material impregnated with a butadiene phenolic resin (Conventional - 1) as compared to a fibrous base material impregnated with a silicone-phenolic resin blend (Example C) and a fibrous base material impregnated with different epoxy-phenolic resins (Example D 0.016 inch (0.41 mm) thin lining and Example F 0.020 inch (0.51 mm) thick lining). Fig. 6 is a graph showing the ratio of static to dynamic coefficient of friction performance as the number of cycles increases for the Conventional - 1 material as compared to the Examples C, D and F materials. Fig. 7 is a graph showing the dynamic coefficient of friction performance as the number of cycles increases for the Conventional - 1 material as compared to the Examples C, D and F materials. Fig. 8 is a graph showing the dynamic mid point coefficient friction performance as the number of cycles increases for the Conventional - 1 material as compared to Examples B, D and a fibrous base material impregnated with a different epoxy phenolic resin (Example E). Fig. 9 is a graph showing the stop time performance as the number of cycles increases for the Conventional - 1 material as compared to the Examples B, D and E materials. Fig. 10 is a graph showing high energy friction test cycles for a conventional material impregnated with a phenolic resin (Conventional - 1) as compared to fibrous base material impregnated with an epoxy-phenolic resin (Example D). Fig. 11 is a graph showing the high speed durability test at 7,000 rpm, 0.3 LPM oil flow 1.5 kg-cm-sec 2 inertia showing the dynamic coefficient of friction as the number of cycles increases for a fibrous base friction material impregnated with an epoxy-phenolic resin (Example D) and the Conventional - 1 material and a conventional friction material impregnated with a phenolic resin (Conventional - 2). Fig. 12 is a graph showing the high energy durability test at 3,600 rpm, 8.0 kg/cm 2 lining pressure, 5.0 kg-cm-sec 2 inertia showing the dynamic coefficient of friction as the number of cycles increases for the Example D and the two conventional materials, Conventional - 1 and Conventional - 2. Fig. 13 is a graph showing the engine dynamometer 4-3 down shift durability test, 2,000 cc IG/FE engine, 5,800 rpm showing the shift time in seconds for the 4-3 down shift engagements for the Example D and the two conventional friction materials, Conventional - 1 and Conventional - 2. Fig. 14 is a graph comparing the shear strength (psi) for a fibrous base material impregnated with an epoxy-phenolic resin (Example E) and a conventional material (Conventional - 2). Fig. 15 is a graph showing the pore size (in microns) for a fibrous base material impregnated with an epoxy-phenolic resin (Example E) and a conventional material (Conventional - 2). Fig. 16 is a graph comparing the liquid permeability (cm 2 x 10 -3 ) for a fibrous base material impregnated with an epoxy-phenolic resin (Example E) and a conventional material (Conventional - 2). Fig. 17 is a graph showing the speed, torque, temperature and applied pressure for Example E at an interface temperature of about 695°F (368°C) for 500 cycles. Fig. 18 is a graph showing the speed, torque, temperature and applied pressure for Example E at an interface temperature of about 896°F (480°C) for 10,500 cycles. Fig. 19 is a graph showing the mid point dynamic coefficient of friction for Example E as the number of cycles increases. Fig. 20 is a graph showing the high speed durability showing the mid point coefficient of friction as the number of cycles increases for Examples C and E, as compared to the Conventional - 1 material. Fig. 21 is a graph showing a high speed durability at 6,000 rpm using an Exxon 1975 fluid showing the static to dynamic coefficient of friction ratio as the number of cycles increases for Examples C and E, as compared to the Conventional - 1 material. Fig. 22 is a graph showing a high speed durability test at 6,000 rpm using an automatic transmission fluid JWS2318K showing the coefficient of friction as the number of cycles increases for Examples C, D and F, as compared to the Conventional - 1 material. Various resins useful in the present invention include phenolic resins and phenolic-based resins. It is to be understood that various phenolic-based resins which include in the resin blend other modifying ingredients, such as epoxy, butadiene, silicone, tung oil, benzene, cashew nut oil and the like, are contemplated as being useful with the present invention. In the phenolic-modified resins, the phenolic resin is generally present at about 50% or greater by weight (excluding any solvents present) of the resin blend. However, it has been found that friction materials, in certain embodiments, can be improved when the impregnant resin blend contains about 5 to about 80%, by weight, and for certain purposes, about 15 to about 55%, and in certain embodiments about 15 to about 25%, by weight, of silicone resin based on the weight of the silicone-phenolic mixture (excluding solvents and other processing acids). Silicone resins useful in the present invention include, for example, thermal curing silicone sealants and silicone rubbers. Various silicone resins are useful with the present invention. One resin, in particular, comprises xylene and acetylacetone (2,4-pentanedione). The silicone resin has a boiling point of about 362°F (183°C), vapor pressure at 68°F mm, Hg: 21, vapor density (air = 1) of 4.8, negligible solubility in water, specific gravity of about 1.09, percent volatile, by weight, 5% evaporation rate (ether=1), less than 0.1, flash point about 149°F (65°C) using the Pensky-Martens method. It is to be understood that other silicone resins can be utilized with the present invention. Other useful resin blends include, for example, a suitable phenolic resin comprises (% by wt.): about 55 to about 60% phenolic resin; about 20 to about 25% ethyl alcohol; about 10 to about 14% phenol; about 3 to about 4% methyl alcohol; about 0.3 to about 0.8% formaldehyde; and, about 10 to about 20% water. Another suitable phenolic-based resin comprises (% by wt.): about 50 to about 55% phenol/formaldehyde resin; about 0.5% formaldehyde; about 11% phenol; about 30 to about 35% isopropanol; and, about 1 to about 5% water. It has also been found that another useful resin is an epoxy modified phenolic resin which contains about 5 to about 25 percent, by weight, and preferably about 10 to about 15 percent, by weight, of an epoxy compound with the remainder (excluding solvents and other processing aids) phenolic resin. The epoxy-phenolic resin compound provides, in certain embodiments, higher heat resistance to the friction material than the phenolic resin alone. It further contemplated that other ingredients and processing aids known to be useful in both preparing resin blends and in preparing impregnating fibrous-based materials can be included in the friction materials. For the embodiments where a phenolic resin and silicone resin are used, no new compound is formed when the silicone resin and phenolic resin are blended together. Table 1 shows the prominent FT-IR peaks in wave numbers for a cured silicone resin, a cured phenolic resin, and about 50/50 blend of silicone resin and phenolic resin which has been cured. As can be seen, no new peaks occur in the 50/50 silicone-phenolic blend, and the peaks that are present reflect the presence of both the silicone resin and the phenolic resin. Thus, it is shown that the resins cure separately and that no new compound is formed. PROMINENT FT-IR PEAKS IN WAVENUMBERS SILICONE RESIN PHENOLIC RESIN 50/50 BLEND ---- 3364 3366 2966 ---- 2964 ---- 1510 1510 ---- 1479 1481 1412 ---- 1410 1271 ---- 1261 798 ---- 800 767 ---- 769 Both the silicone resin and the phenolic resin are present in solvents which are compatible to each other. These resins are mixed together (in preferred embodiments) to form a homogeneous blend and then used to impregnate a fibrous base material. There is not the same effect if a fibrous base material is impregnated with a phenolic resin and then a silicone resin is added thereafter or vice versa. There is also a difference between a mixture of a silicone-phenolic resin solution, and emulsions of silicone resin powder and/or phenolic resin powder. When silicone resins and phenolic resins are in solution they are not cured at all. In contrast, the powder particles of silicone resins and phenolic resins are partially cured. The partial cure of the silicone resins and the phenolic resins inhibits a good impregnation of the fibrous base material. Therefore, according to one aspect of the present invention, the fibrous base material is impregnated with a blend of a silicone resin in a solvent which is compatible with the phenolic resin and its solvent. In one embodiment, isopropanol has been found to be an especially suitable solvent. It is to be understood, however, that various other suitable solvents, such as ethanol, methyl-ethyl ketone, butanol, isopropanol, toluene and the like, can be utilized in the practice of this invention. According to the present invention, the presence of a silicone resin, when blended with a phenolic resin and used to impregnate a fibrous base material, causes the resulting friction materials to be more elastic than fibrous base materials impregnated only with a phenolic resin. When pressures are applied to the silicone-phenolic resin blended impregnated friction material of the present invention, there is a more even distribution of pressure which, in turn, reduces the likelihood of uneven lining wear. After the silicone resin and phenolic resin are mixed together, the mixture is used to impregnate a fibrous base material. Various methods for impregnating materials can be practiced with the present invention. The fibrous base material is impregnated with the phenolic or modified phenolic resin, preferrably so that the impregnating resin material comprises about 45 to about 65 parts, by weight, per 100 parts, by weight, of the friction material. After the fibrous base material has been impregnated with the resin, the impregnated fibrous base material is heated to a desired temperature for a predetermined length of time to form the friction material. The heating cures the phenolic resin at a temperature of about 300°F (149°C). When other resins are present, such as a silicone resin, the heating cures the silicone resin at a temperature of about 400°F (204°C). Thereafter, the impregnated and cured friction material is adhered to the desired substrate by suitable means. Another aspect of the present invention relates to a fibrous base material comprising less fibrillated aramid fibers, synthetic graphite and at least one filler material, which are combined to form a paper-like fibrous base material. It is to be understood that various methods of forming fibrous base materials are contemplated as being useful in preparing the fibrous base material of the present invention. It has been found by the inventors herein that the use of less fibrillated aramid fibers and synthetic graphite in a fibrous base material improves the friction material's ability to withstand high temperatures. While various friction lining materials disclose the use of aramid fibers, it has not been known until the present invention to provide a friction material comprising less fibrillated aramid fibers which generally have few fibrils attached to a core fiber. The use of the less fibrillated aramid fibers provides a friction material having a more porous structure; i.e., there are more and larger pores than if a typical fibrillated aramid fiber is used. The porous structure is generally defined by the pore size and liquid permeability. The length of the less fibrillated fiber ranges from about 0.5 to about 6 mm and has a Standard Canadian Freeness of greater than about 450 and in certain embodiments, about 500 to about 550. The "Canadian Freeness Standard" (T227 om-85) means that the degree of fibrillation of fibers can be described as the measurement of freeness of the fibers. The Canadian freeness test is an empirical procedure which gives an arbitrary measure of the rate at which suspension of three grams of fibers in one liter of water may be drained. Therefore, the less fibrillated aramid fibers have higher freeness or higher rate of drainage of fluid from the friction material than other aramid fibers or pulp. For example, more fibrillated fibers, such as Kevlar® aramid pulp, have a freeness of about 285-290. The more porous the structure of the friction material, the more efficient is the heat dissipation. The oil flow in and out of the friction material during engagement of the friction material during use occurs more rapidly when the friction material is porous. It has further been discovered that the less fibrillated fibers, synthetic graphite and filler improve the pore structure of the fibrous base material so that there are more porous openings throughout the fibrous base material. The increased porosity also increases the elasticity of the friction material. A lower degree of fibrillation of the less fibrillated aramid fibers results in a friction material having a more porous structure. It has not been known until the present invention to include synthetic graphite in a fibrous base material comprising less fibrillated aramid fibers. The use of synthetic graphite in the fibrous base material provides a more three dimensional structure to the fibrous base material than other types of graphite material. The synthetic graphite is made by graphitization of a raw stock material such as petroleum coke and a coal tar pitch binder. The raw materials are mixed and heated to temperatures of about 2,800 to about 3,000°C (1538 to 1649°C) in special graphitizing furnaces which convert the baked carbon body into a polycrystaline graphite article. The synthetic graphite (which has high thermal conductivity) provides the friction material with the ability to dissipate heat more rapidly than other types of graphite. In certain embodiments, it is preferred that the size and geometry of the synthetic graphite be in the about 20 to about 50 micron size range. In these certain embodiments, it has been discovered that if the graphite particle size is too large or too small, there is not the optimum three-dimensional structure and consequently the heat resistance is not as optimum. Various fillers are also used in the fibrous base material of the present invention. In particular, silica fillers, such as diatomaceous earth, are useful. However, it is contemplated that other types of fillers are suitable for use in the present invention and that the choice filler depends on the particular requirements of the friction material. Other ingredients can be added to the fibrous base material of the present invention, including for example, cotton fibers which can be added to give the fibrous material higher coefficients of friction. In certain embodiments, about 0 to about 20%, and in certain embodiments about 5 to about 15%, other filler such as aramid pulp and/or aramid floc can also be added to the fibrous base material. One example of a formulation for a fibrous base material comprises about 10 to about 50%, by weight, of a less fibrillated aramid fiber; about 10 to about 35%, by weight, of a synthetic graphite; and, about 20 to about 45%, by weight, of a filler material. In certain embodiments, one particular formulation has found to be useful comprises about 45 to about 50%, by weight, less fibrillated aramid fibers: about 15 to about 25%, by weight, synthetic graphite; and, about 20 to about 30%, by weight, filler. Another useful formulation comprises about 20 to about 30% less fibrillated aramid fibers, about 15 to about 25% synthetic graphite, about 20 to about 30% filler material, and optionally about 0 to about 40% cotton fibers. In further embodiments, the cotton fibers can be present at about 20 to about 40%, by weight, or about 25 to about 35%, by weight. The following examples provide further evidence that the fibrous base material and friction material of the present invention are an improvement over the conventional friction material. Various preferred embodiments of the invention are described in the following examples, which however, are not intended to limit the scope of the invention. Examples A and B both are a fibrous base material comprising about, in percent, in weight, about 45% less fibrillated aramid fibers, about 23% synthetic graphite, about 27% diatomaceous earth filler, and about 5% optional filler comprising aramid pulp. Example A is impregnated with a phenolic material and Example B is impregnated with a silicone-phenolic resin blend comprising about 20% silicone and about 80% phenolic resins. Example C is a fibrous base material comprising in percent, by weight, about 35% less fibrillated aramid fibers, about 25% synthetic graphite, about 25% diatomaceous earth filler material, and other optional fillers of about 5% aramid pulp and about 10% aramid floc, and impregnated with a silicone-phenolic resin blend. Example D is a fibrous base material comprising in percent, by weight, about 25% less fibrillated aramid fibers, about 20% synthetic graphite, about 25% diatomaceous earth, and about 30% cotton fibers and impregnated with a first epoxy-phenolic resin blend comprising about 10% epoxy and about 90% phenolic resins. Example E is a fibrous base material comprising about 25% less fibrillated aramid fibers, about 20% synthetic graphite, about 25% diatomaceous earth, and about 30% cotton fibers and impregnated with a second epoxy-phenolic resin. Example F is a fibrous base material comprising, in percent, by weight, about 25% less fibrillated aramid fibers, about 20% synthetic graphite, about 25% diatomaceous earth, and about 30% cotton fibers and impregnated with the second epoxy-phenolic resin blend Example 1 Fig. 1A, shows a scanning electron microscopic (SEM) photograph of Example C which indicates that a thin film of silicone resin forms between the fibers during impregnation. Example C has an increased pore size over friction materials impregnated with either a silicone resin or phenolic resin alone. Since the silicone resins and phenolic resins cure at different temperatures, the phenolic resin cures first while the silicone resin cures later. A thin film of silicone resins formed between the fibers during cure. This thin film of silicone resin between the fibers is thought to contribute to the high friction stability of the friction material. The film of silicone resin slows down the deterioration of the friction material and allows the friction material to have a high heat resistance at high temperatures. The SEM photographs shown in Fig. 1A show a much larger pore structure than for the phenolic resin-impregnated friction material, a conventional material (Conventional - 1) which contains no less fibrillated aramid fibers and no synthetic graphite, shown in Fig. 1B. As seen in Fig. 1 A, the blend of silicone and phenolic resins results in a fiber-resin interaction which creates a flexible and open fiber network. Up to about a 50% larger pore size has been seen with the phenolic-silicone blend impregnated friction material than over phenolic resin impregnated friction material alone. In certain embodiments, the mean pore size ranges from about 2.5 to about 4 microns in diameter and the friction material had readily available air voids of at least about 50% and in certain embodiments at least about 60% or higher. Example 2 The capillary flow and permeability tests are shown in Table 2 below for Examples B, D, E and a comparative material having natural graphite but no synthetic graphite. The higher mean flow pore diameter and Darcy's permeability indicate that the friction material is more likely to run cooler or with less heat generated in a transmission due to better automatic transmission fluid flow of material throughout the porous structure of the friction material. During operation of a transmission system, oil deposits on the surface of a friction material tend to develop over time due to a breakdown of the automatic transmission fluid, especially at high temperatures. The oil deposits on the fibers decrease the pore openings. Therefore, when a friction material initially starts with larger pores, there are more open pores remaining during the useful life of the friction material. In addition, the silicone resin, due its elastic characteristics, allows the fibers in the friction lining to have a more open structure. CAPILLARY FLOW AND PERMEABILITY Darcy's Permeability Mean Flow Pore Diameter Sample Thickness Inches, cm Ex. B 2.0x10 2.77 microns 0.021 0.05334 Ex. D 1.0x10 -2 2.85 microns 0.016 0.04191 Ex. E 1.0x10 -2 2.34 microns 0.017 0.04318 Compar. 5.1x10 -3 1.77 microns 0.019 0.04826 Example 3 Glaze analysis of the scanning electron microscopic photographs shows that the silicone-phenolic resin blend has a slight fiber compression on the surface while the phenolic resin alone has a pronounced fiber compression on the surface for unused plates. Further, as seen in Table 3, in used plates, there are open pores remaining in a silicone-phenolic resin blend while there are very few pores open in the phenolic resin material alone. GLAZE ANALYSIS SCANNING ELECTRON MICROSCOPY UNUSED PLATES Example C Conventional Material - 1 * Slight fiber compression on surface * Pronounced fiber compression on surface * No fiber compression internally * No fiber compression internally * Resin forms a film between fibers * Resin only coats fibers USED PLATES Example C Conventional Material - 1 * Surface Glazes * Surface glazes * Open pores * Very few open pores Example 4 Previously, unreacted silicone resins have not been acceptable for use in a friction material since the silicone resin has low strength. However, it has now been found that the shear strength of the silicone-phenolic resin blends is remarkably higher than for phenolic resins alone. The tensile shear test was run on the Instron tensile tester. A modified lap shear configuration was used with a 2 square inch area of friction material bonded on both side to steel plates. This assembly was then pulled until the paper sheared. The values in Table 4 below indicate the internal shear strength of the paper under dry conditions at room temperature for Examples B, E and D. The higher the shear strength, the better mechanical strength the friction material has which means that more pressure is needed to shear the friction lining. Shear Strength PSI (MPa) Ex. B 382.5 (2.64) 382.5 (2.64) Ex. E 325.0 (2.24) 290.0 (2.00) Ex. D 352.5 (2.43) 385.0 (2.65) Example 5 The silicone-phenolic resin blend provides at least about a 50% increase in the "burn off" temperature of the friction material. This high friction stability is an advantage over the currently available friction materials. A thermal gravimetric analysis (TGA) shown in Fig. 2, wherein the TGA curve shifts to higher temperatures, indicates increased heat resistance of the silicone-phenolic resin blend over the phenolic material. Both Examples A and B have improved heat resistance over conventional materials and Example B is especially suitable for end-use friction material applications where heat resistance is a critical criterion. Figs. 3 and 4 compare the TGA graphs shown in Fig. 2, and the change in derivative weight (%/°C) for the phenolic resin, Example A in Fig. 2 (Fig. 3) and the silicone-phenolic blend, Example B in Fig. 2 (Fig. 4). The percent change in weight for the phenolic resin was 69.41% while the percent change in weight for the silicone-phenolic blend was 61.87%. As seen from Figs. 3-4, the more rapid the weight loss, the less heat resistance the friction material possess. Example 6 Figure 5 shows the stop time as the number of cycles increases for various materials: Example C, D and F as compared to the Conventional - 1 material impregnated with a butadiene-phenolic resin. The fibrous materials (Examples C, D and F) maintained a relatively uniform stop time, while the stop time for the conventional material rapidly rose to unacceptable levels. The ratio between the static coefficient of friction and the dynamic coefficient of friction as the number of cycles increases was compared for Examples C, D and F and for the Conventional - 1 material. As can be seen in Fig. 6, the fibrous base material impregnated with silicone-phenolic blend material (Example C) performs consistently better than the conventional material while the fibrous base material impregnated with epoxy-phenolic resins (Examples D and F) performed comparatively well. The dynamic coefficient of friction as the number of cycles increase was compared for Examples C, D and F and for the conventional material (Conventional - 1). Fig. 7 shows the dynamic coefficient of friction for the friction materials (Examples C, D and F) remain relatively steady as the number the cycles increased. Thus, the fibrous base materials perform much better at a high speed than the conventional material. It is important to note that there is no "fall off" of the coefficient of friction as the number of cycles increases for the fibrous base materials. A materials evaluation for a clutch run at 6,600 rpm (65m/sec.), limited lubrication of 0.2 gpm was conducted for Examples B, D and E and for the Conventional - 1 material. The dynamic mid point coefficient graphs of Fig. 8 shows that the conventional material was totally unacceptable while the friction Examples B, D and E materials have a relatively steady coefficient of friction indicating that the system was very stable. As can be seen in Fig. 9, the stop time for the conventional material rapidly increased to unacceptable levels while the friction materials (Examples B, D and E) maintained an acceptably short stop time of about 0.52 to about 0.58 seconds throughout the test. Example 7 In certain embodiments, it is preferred that the target pick up of resin by the friction material range from about 40 to about 65%, and, in certain embodiments, about 60 to at least 65%, by weight, total silicone-phenolic resin. After the fibrous base material is impregnated with the resin, the fibrous base material is cured for a period of time (in certain embodiments for about 1/2 hour) at temperatures ranging between 300-400°C to cure the resin binder in the friction material. The final thickness of the friction material depends on the initial thickness of the fibrous base material and, in certain embodiments, preferably ranges from about 0.014" to about 0.040". In Table 5 below, a friction material comprising the fibrous base material impregnated with about 60% resin pick up (P.U.) of a silicone-phenolic resin (Example C) was compared to a friction material comprising the same fibrous base material as in Example C but impregnated with a phenolic resin with about 60% resin pick up (P.U.) (Example C-1) and to the Conventional - 1 material impregnated with a phenolic resin with about 49% P.U. (Conventional - 1). Assembly or core plates were lined with friction materials impregnated with the tested resins to form a pack for testing. The dynamic coefficient of friction remained steady (with a loss of only 5%) as the number of cycles increased for the silicone-phenolic resin friction materials. There was no lining wear on the plates using the silicone-phenolic resin friction material. The lining condition of the silicone-phenolic resin blend friction material remained good without breakouts, abrasion, or glazing occurring. Further, the steel condition of the separator plates show no hot spots for the silicone-phenolic blend friction materials. EFFECT OF RESIN CHANGE TEST RESIN % CONV'L - 1 49% P.U. EX. C-1 60% P.U. EXAMPLE C 60% P.U. LINING THICKNESS CYCLES 0.016'' (0.41 mm) 0.016'' (0.41 mm) 0.016'' (0.41 mm) MID. DYNAMIC 75 0.135 0.134 0.134 3,000 0.121 0.123 0.130 6,000 0.118 0.113 0.127 STOP TIME SEC. 75 0.804 0.799 0.796 3,000 0.880 0.858 0.817 6,000 0.904 0.910 0.835 TORQUE CURVE SHAPING Decrease Decrease Decrease LINING LOSS PER PLATE 0.0027'' (69 µm) 0.0009'' (23 µm) No Loss LINING CONDITION Breakouts Heavy Abrasion Good Glaze Glaze STEEL CONDITION Distinct Hot Few Small Light Heat Spots Hot Spots Stains Example 8 Table 6 below shows compression/relaxation studies done on an MTS machine. This test reports the effect on paper caliper caused by repeatedly pressing on a sample and releasing the sample through a series of different pressures. These readings provide an indication of the internal resistance to set or compacting due to pressing. The Example B material shows a greater elasticity than the comparative example described in Table 2 above. This greater elasticity allows for more uniform heat dissipation during use of the friction material since the fluid in the transmission or brake can rapidly move through the porous structure. Further, the increased elasticity provides more uniform pressure or even pressure distribution on the friction material such that uneven lining wear or separator plate "hot spots" are eliminated. LOAD VS DEFLECTION Compression/compression Set Pressure Psi (MPa) Example B Compar. 15 psi (0.1 MPa) .0000"/1 in. (0 mm/25.4 mm) .0000"/1 in. (0 mm/25.4 mm) .0000" (0 mm) .0000" (0 mm) 50 psi (0.34 MPa) .0180"/1 in. (0.46 mm/25.4 mm) .0104"/1 in. (0.26 mm/25.4 mm) .0066" (0.17 mm) .0034" (0.09 mm) 100 psi (0.69 MPa) .0348"/1 in. (0.88 mm/25.4 mm) .0233"/1 in. (0.59 mm/25.4 mm) .0083" (0.21 mm) .0049" (0.12 mm) 200 psi (1.38 MPa) .0600"/1 in. (1.52 mm/25.4 mm) 0.419"/1 in. (1.06 mm/25.4 mm) .0115" (0.29 mm) .0070" (0.18 mm) 300 psi (2.07 MPa) .0805"/1 in. 2.05 mm/25.4 mm) .0565"/1 in. (1.44 mm/25.4) .0123" (0.31 mm) .0076" (0.19 mm) 400 psi .0963"/1 in. (2.45 mm/25.4 mm) .0658"/1 in. (1.67 mm/25.4 mm) .0159" (0.40 mm) .0070" (0.18 mm) 500 psi .1112"/1 in. (2.82 mm/25.4 mm) .0742"/1 in. (1.88 mm/25.4 mm) .0188" (0.48 mm) .0079" (0.20 mm) 700 psi .1369"/1 in. (3.48 mm/25.4 mm) .0939"/1 in. (2.39 mm/25.4 mm) .0232" (0.59 mm) .0111" (0.28 mm) 900 psi .1533"/1 in (3.89 mm/25.4 mm) .1090"/1 in. (2.77 mm/25.4 mm) .0242" (0.61 mm) .0134" (0.34 mm) 1100 psi .1703"/1 in. (4.32 mm/25.4 mm) .1248"/1 in. (3.17 mm/25.4 mm) .0267" (0.68 mm) .0152" (0.39 mm) 1300 psi .1922"/1 in. (4.88 mm/25.4 mm) .1419"/1 in. (3.60 mm/25.4 mm) .0324" (0.82 mm) .0190" (0.48 mm) 1500 psi .2179"/1 in. (5.53mm/25.4) .1 630"/1 in. (4.14mm/25.4mm) .0404" (1.03mm) .0248" (0.63mm) Example 9 A friction material comprising less fibrillated aramid fibers and synthetic graphite impregnated with an epoxy modified phenolic resin (Example D) and was compared to the conventional material (Conventional - 1). A high speed friction cycles test is shown in Fig. 10, comparing the stroking test life and high energy friction test cycles. The friction material of the present invention performs better in all aspects than the conventional friction material. Fig. 11 shows the results of a high speed durability test at 7,000 rpm, 0.3 LPM oil flow with 1.5 kg-cm-sec 2 inertia. As the number of cycles increases, the dynamic coefficient of friction remained relatively uniform for the friction material (Example D) while one conventional material (Conventional - 2) failed at the beginning of the test and the performance of another conventional material impregnated with a phenolic-based resin (Conventional - 1) rapidly fell off after about 3,000 cycles. Fig. 12 shows the results of a high energy durability test at 3,600 rpm, 8.0 kg/cm 2 lining pressure at 5.0 kg-cm-sec 2 inertia. The dynamic coefficient of friction for the friction material (Example D) remained remarkably steady throughout the entire durability test. In comparison, the conventional materials failed at an unacceptably short cycle of usage life. Fig. 13 shows the results of an engine dynamometer 4-3 down shift durability test for a 2,000 cc IG/FE engine at 5,800 rpm. As can be seen, the shift time in seconds for the 4-3 down shift engagements for the friction material (Example D) remain relatively constant through at least 40,000 down shift engagements. The conventional materials had rapid increases in shift time at low shift engagement cycles. Example 10 The friction material of the present invention has high durability and high delamination resistance. The shear strength (psi) for the friction material of the present invention is greater than for the conventional materials, as seen in Fig. 14. The use of the less fibrillated fibers and the resulting pore structure of the friction material provides increased thermal resistance to the friction material. The fiber geometry not only provides increased thermal resistance, but also provides delamination resistance and squeal resistance. The presence of the synthetic graphite particles and at least one filler material aids in increasing the thermal resistance, maintaining a steady coefficient of friction, and increasing the squeal resistance. Example 11 The average pore size for the friction material of the present invention as compared to the pore size of a conventionally resin impregnated friction material is shown in Fig. 15. The average pore size of the friction lining of the present invention ranges from about 2.5 to about 12 microns and is between about 20 to about 100% larger than for the conventional friction materials. Example 12 The liquid permeability for the friction material of the present invention was compared to a conventional material impregnated with a phenolic resin (Conventional - 2). As seen in Fig. 16, the friction material of the present invention has about a 20% increase in liquid permeability over the conventional materials. Example 13 Fig. 17 shows a friction material (Example D) comprising about 0.02" (0.51 mm) lining with about 44% pick up of a phenolic-epoxy resin at about 380°F (193°C) after 1/2 hour cure at an interface temperature of about 695°F. Fig. 17 compares the speed, torque, temperature and applied pressure of the material run at 500 cycles showing the high friction stability of the friction material of the present invention. Fig. 18 shows the high friction stability of the friction material (Example D) comprising a 0.02" (0.51 mm) lining with about a 44% resin pick up of another phenolic-based resin cured at 380°F (193°C) for 1/2 hour, at an interface temperature of 895°F (479°C). Fig. 18 shows the speed, torque, temperature and applied pressure of the material run for 10,500 cycles. Table 7 below shows the mid point coefficient of friction for the friction material (Example D) shown in Figs. 17 and 18. The coefficient of friction remains relatively steady as the cycles increase, thus showing the high friction stability of the friction material. Also, as shown in Fig. 19, the mid point dynamic coefficient of friction for the above described friction materials in Figs. 17 and 18 show that as the number of cycles increased, the mid point coefficient of friction remained relatively steady. The torque curve shape shows that the friction material of the present invention is especially useful in high speed, high energy and high temperature applications. The total loss of friction material was only about 0.0077 inches (196 µm) and a loss per plate was about .0039 inches (99 µm). The friction material showed a medium glaze and the separator was only light heat stained, thus indicating a high quality friction material which is stable over a long period of time. Example D CYCLES MID COEFFICIENT 50 .132 100 .136 300 .135 500 .131 550 .131 600 .129 900 .124 1200 .122 1500 .121 2500 .121 4500 .122 6500 .121 8500 .123 10500 .126 Example 14 Fig. 20 shows a high speed durability test comparing a conventional phenolic-based material impregnating a conventional friction lining to one embodiment of the friction material of the present material impregnated with a silicone-phenolic resin blend material (Example C) and another embodiment of the friction material of the present invention impregnated with a phenolic-epoxy resin material (Example D). Both the friction materials of the present invention had more stable mid point coefficients of friction than the conventional friction material. A high speed durability test run at 6,000 rpm was conducted comparing the static to dynamic (S/D) coefficient of friction over a number of increasing cycles. As seen in Fig. 21, the conventional phenolic impregnated friction material was compared to a silicone-phenolic impregnated friction material of the present invention (Example C) and epoxy-phenolic impregnated friction material (Example E) of the present invention. The materials of the present invention have favorable static to dynamic coefficient of friction ratios to the conventional material. The coefficient of friction as cycles increase at 6,000 rpm was tested for three samples of the fibrous base material of the present invention, each impregnated with a resin as follows: phenolic-epoxy impregnated resin at a 0.016 inch (0.41 mm) thin fibrous base material (Example D), phenolic-based resin impregnated at 0.020 inch (0.51 mm) thick fibrous base material (Example F), and a silicone-phenolic resin (Example C). As seen in Fig. 22, these fibrous base materials impregnated with the various resins compared favorably to a conventional friction material, which performed more poorly than each of the friction materials of the present invention. The present invention is useful as a high energy friction material for use with clutch plates, transmission bands, brake shoes; synchronizer rings, friction disks or system plates. The above descriptions of the preferred and alternative embodiments of the present invention are intended to be illustrative and are not intended to be limiting upon the scope and content of the following claims.

Claims: A fibrous base material for use in a non-asbestos friction material comprising a plurality of fibrillated aramid fibers and at least one filler material, characterised in that the material further comprises synthetic graphite, and in that the aramid fibers have a freeness of at least 450 on the Canadian Freeness Standard Index. A fibrous base material according to claim 1, characterised in that the fibrillated aramid fiber and synthetic graphite are present in amounts sufficient to provide high heat resistance and substantially uniform coefficient of friction to the friction material. A fibrous base material according to claim 1, characterised in that the synthetic graphite is made by graphitization at temperatures of about 2800-3,000°C and has a size ranging from about 20 to about 50 microns in diameter. A fibrous base material according to claim 1, characterised in that the filler comprises diatomaceous earth. A fibrous base material according to claim 1, characterised in that the fibrous base material defines pores ranging in mean average size from about 2.5 to about 12 microns in diameter. A fibrous base material according to claim 1, characterised by about 10 to about 50%, by weight, fibrillated aramid fiber; about 10 to 35%, by weight, synthetic graphite; and about 20 to about 45% by weight, filler material. A fibrous base material according to claim 1, characterised by in percent, by weight, about 20 to about 30% fibrillated aramid fibers, about 15 to 35% synthetic graphite, about 20 to about 30% filler, and about 0 to about 40% cotton fibers. A friction material comprising a fibrous base material according to claim 7, characterised in that the fibrous base material comprises about 20% to about 40% cotton fibers. A friction material comprising a fibrous base material according to claim 1, characterised by readily available air voids of at least about 50%. A non-asbestos friction material characterised by the fibrous base material according to claim 1 impregnated with phenolic resin or a modified phenolic resin. A non-asbestos friction material according to claim 10, characterised in that the friction material comprises approximately 40 to about 60% resin, by weight. A non-asbestos friction material according to claim 10, characterised in that the modified phenolic resin comprises an epoxy phenolic resin. A non-asbestos friction material according to claim 12, characterised in that the amount of epoxy resin present in the epoxy phenolic resin ranges from about 5 to about 25%, by weight, based on the weight of the epoxy phenolic resin. A non-asbestos friction material according to claim 12, characterised in that the amount of epoxy resin present in the epoxy phenolic resin ranges from about 10 to about 15%, by weight, based on the weight of the epoxy phenolic resin. A non-asbestos friction material, characterised by the fibrous base material according to claim 1 impregnated with a mixture of a phenolic resin and a silicone resin wherein the amount of silicone resin in the mixture ranges from approximately 5 to approximately 80%, by weight, based on the weight of the mixture, the friction material exhibiting high heat resistance and substantially uniform coefficient of friction. A friction material according to claim 15, characterised in that the phenolic resin is present in a solvent material and the silicone resin is present in a solvent material which is compatible with the solvent material of the phenolic resin. A friction material according to claim 15, characterised in that the amount of silicone resin present in the silicone-phenolic resin mixture ranges from about 20 to about 25%, by weight, based on the weight of the mixture. A friction material according to claim 15, characterised in that the amount of silicone resin present in the silicone phenolic resin mixture ranges from about 15 to about 25%, by weight, based on the weight of the mixture. A process for producing a non-asbestos friction material comprising mixing fibrillated aramid fibers having a freeness of at least about 450 on the Canadian Freeness Standard Index, with synthetic graphite and at least one filler to form a fibrous base material, impregnating the fibrous base material with at least one phenolic resin or modified phenolic resin, and thereafter curing the impregnated fibrous base material at a predetermined temperature for a predetermined period of time. A process for producing a non-asbestos friction material characterised by mixing a phenolic resin with a silicone resin, impregnating a fibrous base material according to claim 1 with the silicone-phenolic resin mixture, and thereafter heating the impregnated fibrous base material to cure the phenolic resin and the silicone resin.

Inventors: LAM ROBERT CHI-CHIU, YESNIK MARC A, LAM, ROBERT CHI-CHIU, YESNIK, MARC A.
Applicants: BORG WARNER AUTOMOTIVE, BORG-WARNER AUTOMOTIVE, INC.
Application year: 1994
Main classification: F16D69
IPC codes: C04B41, F16D69, C04B30, C08L61